Advances in analytical technology have pushed the limits of human understanding of chemical and physical phenomena. New tools create the opportunity for the new discoveries. Currently available techniques, such as laser desorption techniques, allow analysis of the chemical composition of surfaces at the micron level. However, conventional laser desorption techniques can be limited in their ability to desorb and ionize analytes present at the surface being analyzed. Thus, there is room for improvement in surface extraction technology.
Electrospray is an alternative to Matrix-assisted laser desorption/ionization (MALDI). Electrospray generally involves flowing a sample liquid into an electrospray ion source comprising a small tube or capillary which is maintained at a high voltage, in absolute value terms, with respect to a nearby surface. The nearby (e.g. 1 cm) surface is commonly referred to as the counter electrode. Conventional ES systems for mass spectrometry apply high voltage (relative to a ground reference) to the emitter electrode while holding the counter electrode at a lower, near ground reference voltage. For the positive ion mode of operation, the voltage on the emitter is high positive, while for negative ion mode the emitter voltage is high negative.
The liquid introduced into the tube or capillary is dispersed and emitted as fine electrically charged droplets (plume) by the applied electrical field generated between the tube or capillary which is held at high voltage, referred to as the working electrode, and the nearby surface. In a typical ES-MS process, a solution containing analytes of interest is directed to the ES emitter which is held at high voltage, resulting in a charged solvent droplet spray or plume. The droplets drift towards the counter electrode under the influence of the electric field. As the droplets travel, gas-phase ions are liberated from the droplets. This process produces a quasi-continuous steady-state current with the charged droplets and ions constituting the current and completing the series circuit. Electrospray (ES) is a method of producing highly charged droplets and gas phase ions. A particularly useful application for electrospray is the production of gas phase ions from analytes in liquid solutions delivered by methods such as high pressure liquid chromatography, capillary electrophoresis or capillary electrochromatography to a system for detection and analysis, such as a mass spectrometer (MS). The electrospray process generally includes flowing an analyte liquid into an electrospray ion source comprising a small tube or capillary which is maintained at a high voltage in absolute value terms, with respect to a nearby surface. The small tube or capillary functions as an emitter electrode. In a typical ES-MS system, a solution containing analytes of interest is pumped through the emitter electrode and sprayed towards the remotely located orifice plate of the mass spectrometer. In this arrangement, the orifice plate functions as the counter electrode. Under the influence of the electric field between the emitter electrode and the orifice plate, ions in solution that are of the same polarity as the voltage applied to the ES capillary buildup an excess charge at the surface of the liquid exiting the emitter until a point is reached where the Coulombic forces are sufficient to overcome the surface tension of the liquid. At this point, droplets enriched in ions of this polarity are emitted from the capillary and drift toward the counter electrode. This process produces a quasi-continuous steady-state electrical current. Several methods for conducting surface sampling for electrospray mass spectrometry analysis, as well as other kinds of analysis, have been developed. Some such systems and methods are shown in US patents and Publications Nos. U.S. Pat. No. 6,803,566; U.S. Pat. No. 7,295,026, US 2010/0002905, and US 2010/0224013. The disclosure of these patents and publications is hereby incorporated fully by reference.